Had an issue installing a graphic card the other day. One screen on a wall facing away from me and the computer screen. When the big screen was plugged in the pc screen just had the wallpaper. So I worked out that the desktop was extended and changed it to clone mode. This was difficult as I had to get someone to view the screen on the other side of the wall to tell me where the mouse was! I couldn’t configure it without the screen plugged in.